Many methods are known which normally are computer-implemented methods for computing the design, that is the geometry of an optical lens having a lens front surface and a lens back surface, especially of a spectacle lens having a spectacle lens front surface and a spectacle lens back surface in order to obtain the desired imaging characteristics or to come at least very close to these desired imaging characteristics. In other words, the invention is directed to a method for computing the design of a spectacle lens in order to obtain the specific configuration of the two surfaces of the spectacle lens, namely the back surface which is adapted to be pointing in the direction of the eye of the spectacles wearer, and the spectacle lens front surface which is provided for aligning toward the object side, in order to obtain specific optical characteristics. When it comes to the specific configuration of a surface of a spectacle lens, one also speaks of the surface design of a spectacle glass, that is a spectacle lens. In very general terms, computation methods for the spectacle lens design are known which model the wave characteristics or the particle characteristics of the light. For computing the design of spectacle lenses, numerical light beam tracing methods, so-called ray-tracing algorithms, are normally used. The article entitled “Konzeption und Entwicklung von Gleitsichtgläsern” by Werner Köppen in the publication Deutsche Optiker Zeitschrift DOZ of October 1995, pages 42 to 45, gives a first impression of the complexity of such computations.
The spectacle lens geometry in all of the above-mentioned methods is computed on the basis of monochromatic light, that is a light having only one wavelength. For this wavelength, the so-called design wavelength, the index of refraction is determined for the material used for making the spectacle lens in correspondence to the material-dependent dispersion characteristics. This index of refraction will be incorporated directly into the computation method.
Virtually each spectacle glass (the term “glass” has established itself independently of the material used and its structure) is used in a polychromatic environment, however. The human eye is capable of perceiving light having wavelengths in the range of approximately 380 nm to 750 nm. The maximum of perception of the human eye is at approximately 555 nm (photopic viewing) under daylight conditions, while at night it is approximately at 510 nm (scotopic viewing). Therefore, the total viewing impression is a weighted sum of perceptions of all visible wavelengths.
A spectacle glass is therefore computed or even optimized only for a single wavelength, namely the design wavelength. For this reason, so-called chromatic aberrations are inherent in every conventional spectacle glass. Current computations of spectacle lens geometry provide no visual performance designed for polychromatic light because of the dependency of the sensitivity of the human eye on the wavelength and the ambient brightness.
The term “chromatic aberration” is derived from the Greek “chroma”, i.e. color and the Latin “aberrare”, i.e. to deviate. Chromatic aberrations are imaging errors of optical lenses which are caused due to the fact that light of different wavelength or color is refracted with different intensity. In the case of a converging lens, this leads to different focal lengths for different wavelengths, that is the blue component of the image is focused ahead of the red component.
It is known to correct these errors by a combination of several lenses made of materials of different dispersion. If the wavelengths deviating the most from each other, that is the primary colors red and blue, are brought together, then one speaks of an achromatic correction or an achromatic lens. If, in addition, the primary color green is brought together with the other two colors, then there is an apochromatic correction. However, this correction is only possible in very high-quality and hence very expensive optical systems. Spectacle lenses having an achromatic or apochromatic correction cannot be manufactured at low cost so that this correction is not used in general.
In digital photography, chromatic aberrations can be subsequently corrected with the aid of an electronic image processing in that the different color channels of the image are scaled differently. A correction of this kind can be realized, for example, in a so-called electroactive spectacle lens whose refractive power is locally adjustable with the aid of an electric signal. A conventional spectacle lens, however, does not provide this correction possibility.
Diffractive structures and refractive optical lenses show opposite chromatic behavior. For this reason, combinations of both components can form so-called achromatic (i.e., wavelength-independent) hybrid optics. Whereas in general the geometry of classical lenses is computed with the aid of light beam tracing methods, wavelength methods are generally used to compute diffractive structures. Spectacle lenses configured as hybrid optics are not suitable for the mass market at the present time due to their complex manufacture.
Not only spectacle lenses provided for the correction of the ametropia of the human eye exhibit inherent chromatic aberrations; also, the optically effective components of the human eye itself exhibit chromatic aberrations. That is to say, due to its variable lens, the human eye can adapt the focal length to only one wavelength of the incident light. In part, this phenomenon is directly utilized in the signal detection of the eye because the color receptors are arranged locally separate from each other on the retina in a suitable manner. Combinations of colors impinging on the human eye and having wavelengths which lie very far apart from each other are nonetheless found to be unpleasant.
U.S. Pat. No. 7,677,725 relates to the last-mentioned effect, i.e. to the correction of chromatic aberrations caused by different refractive indices of the materials in the optical components of the human eye for different wavelengths. A method is introduced of how, for example, a spectacle glass in the form of a hybrid optics of refractive optical lens with diffractive surface structure can be computed for minimizing chromatic aberrations of the human eye.